Older women are more likely to give birth to bigger babies, while smaller newborns are more common among younger moms, Dutch researchers say.

Prior studies have hinted at links between a mother’s age and her baby’s birthweight, as well as potential health consequences when babies are very small or very large. Babies who grow less than expected in the womb have a higher risk of birth complications, as well as diabetes and heart disease in adulthood. Very large newborns may be more likely to become obese later in life.

This could be particularly relevant as the age at which women are having children is still increasing in the Western world.

For example, according to a 2010 study by the Pew Research Center, the percentage of U.S. babies being born to women older than 35 went from 9 percent in 1990 to 14 percent in 2008.

Researcher Rachel Bakker of the Erasmus Medical Center in Rotterdam, the Netherlands and her colleagues studied 8,568 Dutch women who gave birth between 2002 and 2006.

The average newborn weighed around 7.7 pounds (3,500 grams). The researchers defined a “small” baby as one weighing 5.5 pounds (2,500 grams) or less and a “large” baby as weighing 10 pounds (4,500 grams) or more.

In total, one of every 20 newborns was small, and another one in 20 was large.

Compared to 30-to-35-year-olds, mothers under 25 tended to be more likely to have small babies. For example, about 4 percent of 30-to-35-year-olds had small newborns, compared to 7 percent of mothers under 20.

On the other hand, older mothers more likely to have large babies, the researchers said in the British Journal of Obstetrics and Gynecology. The risk of a large baby went from 3 percent in the very youngest women, to about 6 percent in 30-to-35-year-olds, to roughly 10 percent in moms over 40.

For the very youngest mothers, the link between age and the risk of delivering a small baby was mostly due to social factors (such as ethnicity, education level, and how many times a woman had given birth before) and lifestyle factors (such as such as diet, smoking and alcohol use).

In the other age groups, social factors- but not lifestyle factors - could also explain why younger women tended to have smaller babies.

But none of those factors could explain why the risk of having a large baby went up in the oldest women.

The findings suggest that other factors in women’s bodies could be playing a role, but the researchers don’t know yet what those might be.

Bakker noted that there might be other factors involved that were not accounted for in the study, and that more research is needed to clarify the range of potential effects.

That means the findings don’t mean a mother’s age alone causes her baby to be born big or small, according to the researchers. Other factors could also affect her newborn’s size, including her weight, her tobacco and alcohol use, and how many other children she’s had.

In the meantime, she told Reuters Health by email, there isn’t enough information available “to advise women about the most optimal age to have children.”

SOURCE: British Journal of Obstetrics and Gynecology, online January 18, 2011.